Monomer applications including, for example, vinyl chloride and styrene monomer processing, require specialized handling to prevent the highly reactive monomer from reacting with the surrounding environment, including pipes, valves, gaskets and the like.
A preferred gasket material for a variety of gasketing applications is polytetrafluoroethylene (PTFE) due to its high corrosion resistance. PTFE gasket material is often reinforced by blending the PTFE with a filler or providing a backing sheet of metallic mesh or foil to add the desired strength characteristics and reduce the creep and cold flow problems associated with PTFE.
However, gaskets made from PTFE-based materials have been shown to fail in monomer applications. Notwithstanding the high corrosion resistance of PTFE, the monomer reacts with the gasket material and causes premature failure of the seal. Failure of PTFE gasket material used in monomer applications is thought to be caused by “pop-corning.” “Pop-corning” occurs when individual monomer units seep into micro-voids in the PTFE structure and polymerize. The individual monomer units are smaller than the micro-voids, thus they can pass around and through the PTFE structure and into these spaces. When a plurality of monomer units migrate into a micro-void, they may polymerize. The polymerized structure often is larger than the sum of the individual monomer units due to the highly organized polymerization. This increase in size of the newly formed polymer within the PTFE micro-void exerts force on the interior walls of the newly formed polymer within the PTFE micro-void exerts force on the interior walls of the micro-void and pushes outward as additional monomers enter, polymerize, and the new polymer expands within the micro-void. This expansion eventually leads to a rupture of the micro-void and a stretching, tearing, or ripping of the surrounding PTFE structure. As this continues throughout the PTFE structure, the gasket material weakens and is more susceptible to failure.
There are several different types of PTFE gasket materials, including modified PTFE, unfilled PTFE, filled PTFE, and coated PTFE systems. When used in monomer applications, all of these PTFE gasket materials are known to exhibit the “pop-corning” effect discussed above.
To overcome the “pop-corning” seen in PTFE based gaskets, a fluorinated ethylene propylene (FEP) or perfluoroalkoxy copolymer (PFA) gasket material is often recommended. These materials do not exhibit the micro-voids seen in the PTFE and, therefore, do not allow monomer to penetrate the surface of the gasket and polymerize. However, fluorinated thermoplastic polymer gasket materials, such as PFA and FEP, do not provide the structural rigidity seen with PTFE gaskets and are likewise susceptible to cold flow/creep. Additionally, these materials are more costly to purchase, more difficult to incorporate with fillers, and require more expensive processing means and machinery.
It is, therefore, desirable to provide a gasket material comprising the rigidity, ease of processing, and relative economy of PTFE with the enhanced resistance to monomers and other highly reactive compounds. It is to these perceived needs that the present invention is directed.